Roughly 700,000 patients die from heart disease in the U.S. each year and 35,000 to 70,000 of these could benefit from mechanical circulatory support or a heart transplant. However, only about 2,500 transplant hearts become available each year. This translates to a profound need for a reliable mechanical blood pump to serve as a cardiac assist device or artificial heart.
Several prior-art devices attempt to solve this problem. Indeed, numerous embodiments of blood pumps exist, but are subject to significant operational problems. Several such prior-art pumps are discussed herein below.
In U.S. Pat. No. 4,688,998 issued to Olsen et al., a motor stator is disclosed that consists of C-shaped rings. The rings substantially increase the diameter of the pump contrary to the anatomical requirement of small size and weight.
In U.S. Pat. Nos. 4,763,032, 4,944,748, 5,078,741, 5,326,344, and 5,385,581, all issued to Bramm et al., a device is disclosed that requires two inflow channels, which increase the total blood-wetted surface. Among other things, this large contact area between artificial materials and the blood increases immune system response to the pump as well as the probability of thromboembolism. Further, connecting the two inlets of this particular pump to the heart is complex and requires additional tubing. Thus, anatomical interference of such pumps with natural organs and structures is increased.
In FIG. 31 of U.S. Pat. No. 4,944,748, there is also disclosed an axial-flow magnetically-levitated blood pump. However, a pump that offers minimal pump volume for a given flow is not disclosed. In particular, the embodiment of FIG. 31 depicts a narrow gap for blood flow between the rotor and the housing. While this is advantageous for magnetic bearing stiffness, it does not lead to a miniaturized pump. FIG. 31 of the '748 patent suggests a ratio of gap to rotor diameter of roughly 1/30 and there is no disclosure of a process for optimally choosing this gap. Nor is there a disclosure of a method of choosing the rotor speed, rotor inertias, and magnetic bearing stiffness in combination to achieve a miniaturized pump.
In U.S. Pat. No. 5,112,202 issued to Oshima et al., a device is disclosed in the form of a centrifugal pump that utilizes a magnetic coupling with mechanical bearings subject to wear. This pump is not suitable for long-term implantation as the bearings will eventually fail due to wear.
In U.S. Pat. Nos. 5,195,877 and 5,470,208 issued to Kletschka, a device is disclosed that requires two inflow channels, which increases the total blood-wetted surface. This large contact area between artificial materials and the blood increases immune system response to the pump. The large surface area also increases the probability of thromboembolism. Further, connecting the two inlets of the pump to the heart is complex and requires additional tubing. Thus, anatomical interference of the pump with natural organs and structures is increased.
In U.S. Pat. No. 5,443,503 issued to Yamane, a pump device is disclosed that has a jewel bearing. Such bearings are subject to wear in a long-term implant. Further, the jewel bearing is a point of blood stasis and is subject to clotting and may lead to thromboembolism. Washout holes are provided to counter such blood stasis, but such washout holes themselves increase the total blood-wetted surface and thus likely negating any benefit.
In U.S. Pat. No. 5,507,629 issued to Jarvik, a device is disclosed that includes a mechanical bearing in the form of a jewel bearing which is a point of blood flow stasis. The blood stasis point is a location of thrombus formation and a source of thromboembolism. Other embodiments of this invention levitate the rotor using only passive magnetic bearings that are inherently unstable, especially during the requisite high-speed rotor rotation. Unstable rotors can contact the pump housing and potentially stop the blood flow.
In U.S. Pat. Nos. 5,695,471 and 5,840,070, both issued to Wampler, a blood pump is disclosed. Wampler '471 is similar to the device of Jarvik '629 in that there is a stasis point at the jewel bearing. The stasis point is a site of thrombus formation and a source of thromboembolism. Further, the jewel bearing will eventually wear out and the impeller will cease to rotate. Wampler '070 uses a hydrodynamic thrust bearing. Such a bearing is highly inappropriate for use within blood processing because such bearings can damage the blood via the high mechanical shear that is inherent to such bearings.
In U.S. Pat. No. 5,725,357 issued to Nakazeki, a device is disclosed in the form of a pump that contains a motor with mechanical bearings subject to wear. Such a device is not suitable for a long-term implant as the mechanical bearings will eventually fail and cause the pump to stop working.
In U.S. Pat. No. 5,928,131 issued to Prem, an elongated pump is disclosed that exposes blood to large regions of foreign material and increases the likelihood of blood damage and thrombus formation. There is also a large region of high blood shear. Blood shear causes blood damage and can trigger undesirable clotting mechanisms in the body. Further, no manner is disclosed for choosing the blood gap, rotor inertias, rotor speed range, and magnetic bearing stiffness to achieve a miniaturized pump.
In U.S. Pat. No. 6,761,532 issued to Capone et al., an axial-flow magnetically-levitated blood pump is disclosed. However, no means for miniaturizing the pump is disclosed. In particular, the embodiment of FIG. 2 of this patent depicts a narrow gap for blood flow between the rotor and the housing. While this is advantageous for magnetic bearing stiffness, it does not lead to a miniaturized pump. FIG. 2 of the '532 patent suggests a ratio of gap to rotor diameter of roughly 1/30 and there is no disclosure of a means for optimally choosing this gap nor is there a disclosure of choosing the rotor speed, rotor inertias, and magnetic bearing stiffness in combination to achieve a miniaturized pump. Moreover, the '532 patent places axial position sensor in such a way as to significantly elongate the pump.
Still further, several Ventricular Assist Device (VAD) systems have been developed over the years for bridge to implant, destination therapy, and as a bridge to recovery. A general understanding of such devices can be gained by reviewing the cardiac assist products of World Heart Inc. of Salt Lake City, Utah or Thoratec Corporation of Pleasanton, Calif.
From the discussion above, it becomes readily apparent that existing devices on the market are overly complex, prone to mechanical failure, promote thromboembolism and strokes, and otherwise suffer from shortcomings related to their ineffective designs. Moreover, none of these designs offer a combination of magnetic levitation and small size. None disclose the use of supercritical operation for miniaturization, nor do they disclose the design of rotor-to-housing gaps in combination with rotor mass, speed ranges, and bearing stiffnesses to achieve supercritical operation and small size. Further, none disclose a cable attachment and internal interconnection space supporting miniaturization.
Accordingly, it is desirable to provide for a new and improved, effective rotary blood pump suitable for long-term implantation into humans for artificial circulatory support. What is needed is such a blood pump that is highly reliable. What is also needed is such a blood pump that meets anatomical requirements with a very compact physical design. What is further needed is such a blood pump that minimizes blood-wetted surface area. Still, what is needed is such a blood pump that minimizes deleterious effects on blood and its circulatory system, the immune system, and other related biological functions. What is also needed is such a blood pump that is not only resilient to everyday accelerations and bodily movements, but also includes stable rotor dynamics, a high motor efficiency, high fluid efficiency, low power consumption for levitation, low vibration, low manufacturing costs, and increased convenience to the patient. Still further, what is needed is a blood pump that overcomes at least some of the disadvantages of the prior art while providing new and useful features.